1. Field of Invention
This invention is directed to detectors for interferometric distance measuring and surface profiling devices.
2. Description of Related Art
Laser interferometers are widely used to measure distances or displacements. One type, polarization interferometers, may form an output beam having two orthogonal, linearly polarized components—a reference beam component and an object beam component. The desired distance or displacement measuring information is carried by the phase difference between the orthogonal, linearly polarized reference and object components of the combined output beam.
Numerous schemes have been devised for measuring this phase difference. In one common scheme, a quadrature-type phase difference detector is used. Quadrature-type phase difference detectors typically divide the combined output beam from the interferometer into multiple beams, typically four beams, and intentionally introduce a different additional phase difference along each beam path. Ideally, the signal derived from each respective beam path corresponds precisely to the phase difference present at the output of the interferometer, plus the additional phase difference introduced along that beam path. The multiple signals are then processed based on trigonometric relationships that are specific to the various intentionally introduced phase differences, in order to determine the phase difference present at the output of the interferometer.
The terms light and radiation are used interchangeably herein. “Light” is not restricted to visible radiation. When used with reference to the operation of a detector, the term input beam generally refers to the orthogonal, linearly polarized reference and object components of an interferometric measurement beam as they encounter the first beam splitting surface of a detector. For example, an interferometric measurement beam output from an interferometer may be rotated by a half wave plate or the like that may be fixed to the detector in front of the first beam splitting surface. In such a case, the input beam is considered to be the interferometric measurement beam after it is rotated by the half wave plate, or the like, as it is presented to the first beam splitting surface of the detector.
The resolution and/or accuracy of a polarization interferometer is often limited by the ability of the phase difference detector, also referred to simply as a detector herein, to generate one or more measurement signals which are truly related only to the phase difference between the object and reference beams output from the interferometer. Conventional configurations of the quadrature-type detectors outlined above typically introduce unwanted “differential phase shifts” between the reference and object beam components that interfere to form the various output signals. In some applications, the measurement errors associated with these differential phase shifts are insignificant and/or unrecognized, and they are ignored. In other applications, “lumped” errors are compensated by special calibrations or adjustments and/or signal processing operations, without considering the root causes of the errors. In any case, such errors may adversely affect the accuracy, complexity, and/or reliability of a detector used for phase difference measurements.
The present invention is directed to a polarization interferometer phase difference detector that uses a novel component configuration and orientation to advance the achievable phase difference measurement accuracy. It should be appreciated that phase difference measurement is already a refined art, and that even small improvements are valued for extending the accuracy and/or reliability of the most accurate measurement technology that is widely available (interferometry). The phase difference measurement is ultimately used to provide a measuring resolution and/or accuracy finer than the basic wavelength of radiation used in the interferometer. For this reason, the degree of meaningful measurement resolution supported by a detector is sometimes referred to as the “interpolation level”. Interpolation levels on the order of 1/100 of a wavelength are not uncommon. A detector according to this invention may provide meaningful interpolation well beyond this level, on the order of 1/1000 of a wavelength, for example.
As previously indicated, conventional detector arrangements typically include errors that arise because, in practice, conventional detectors may introduce unwanted differential phase shifts between the reference and object beam components that interfere to form the various output signals. Such differential phase shifts may directly alter signal amplitudes and the apparent phase difference between the interfering components of the reference and object beams in the outputs. They may also contribute to errors that alter the ideal phase relationships between the various outputs of the detector. In a quadrature-type detector, such phase relationship errors are referred to as “orthogonality errors.”Orthogonality errors arise from the output signals having a phase relationship other than the expected 0, 90, 180 and 270 degree relationships, as assumed in various signal processing operations described below.
In conventional detectors, beamsplitter surfaces and/or coatings may generally be a source of differential phase shifts between the p and s polarization components of the beams with which they interact. We denote the p-s phase shift difference (the induced differential phase shift) in transmission asΔTP-S≡δTP−δTS  (1)and the differential phase shift in reflection asΔRP-S≡δRP−δRS  (2)
where δTP, δTS, δRP, and δRS are the beamsplitter coating/surface induced phase shifts for the p and s components in transmission and reflection respectively. It is generally difficult and/or prohibitively expensive to fabricate beamsplitter coatings that simultaneously control both of transmissive and reflective differential phase shifts as well as the desired ratio between the transmitted and reflected beam intensities.
According to one feature of this invention, it is not necessary to control both the transmissive and reflective differential phase shifts at each beam splitting surface in a detector, in order to eliminate their potential error contributions. Rather, components, surface orientations, and beam polarization directions are arranged such that as the reference beam and object beam components traverse various surfaces and/or coatings, induced phase shifts due to the surface and/or coatings interactions are made to be as similar as possible between the s-components of both the reference beam and object beam, and various output signals are formed by mixing these s-components exclusively. Similarly, surface and/or coatings interactions are made to be as similar as possible between the p-components of both the reference beam and object beam, and various output signals are formed by mixing these p-components exclusively. Thus, the majority of phase shifts induced at the various surfaces and/or coating in a detector according to this invention are arranged to be “common mode” phase shifts in the interference signals that are output from the detector. Such common mode phase shifts do not contribute a net error in the phase difference represented in the interference signals, resulting in a more accurate measurement of the phase difference between the reference beam and object beam components. The resulting improvements in accuracy are important when attempting to increase the measurement resolution and accuracy of an interferometer to smaller fractions of a wavelength of light. In addition, the reduction and/or elimination of differential phase shifts within a detector is achieved in a particularly robust, simple and cost-effective manner in various embodiments according to this invention.
However, despite the foregoing feature of the invention, in the absence of additional care, errors may still arise in association with phase-shifting elements used in a detector. For example, quadrature-type detectors generally include quarter wave elements to introduce a 90 degree phase shift between the object and reference beams along one optical path through the detector. In general, without special care, there may be a differential phase shift ΔTP-S, or ΔRP-S, between the p and s components of the object and reference beams at a beam splitting surface before they reach such a phase-shifting element. Such differential phase shifts, in effect, make the reference and object beams elliptically polarized. Therefore, in general, phase error components will be present along orthogonal polarization directions in the beam(s) transmitted by the phase-shifting element. In such a case, the resulting interference signals used for determining a phase difference will not be the ideal signals desired.
According to a separate feature of the invention, if a phase shifting element is located to receive light transmitted by a beam splitting surface, then it is sufficient if ΔTP-S is adjusted to be insignificant before the transmitted light reaches the phase shifting element. Similarly, if a phase shifting element is located to receive light reflected by a beam splitting surface, then it is sufficient if ΔRP-S is adjusted to be insignificant. This overcomes the previously indicated difficulty of simultaneously controlling both ΔTP-S and ΔRP-S, and makes it practical to provide the desired adjustment of ΔTP-S or ΔRP-S, as well as providing a desired intensity splitting ratio, in a beam spitting coating. In some embodiments, the beam splitter surface coating may also include component layers that provide the phase-shifting element.
In general, in conventional beam splitter coatings, ΔRP-S may be significantly larger than ΔTP-S. Therefore, while adjusting ΔRP-S may be a practical alternative, it may be more reliable and less expensive to adjust ΔTP-S to an insignificant level using a beam splitting coating. Accordingly, a detector may be configured such that all phase shifting elements receive beams transmitted at beam splitting surfaces where ΔTP-S is adjusted to insignificance. Errors that may otherwise arise due to uncontrolled ΔRP-S may then be negated by other previously described features of the invention.
Alternatively to providing a differential phase-shift compensating element integrated with a beam splitter surface coating, a differential phase-shift compensating element may be positioned along a path after a conventional beam splitting surface and before the phase-shifting element that provides the desired phase shift between the reference and object beams. The differential phase-shift compensating element has a design and orientation that compensates for any differential phase shift at the conventional beam splitting surface. It outputs orthogonal, linearly polarized reference and object beams to the following phase-shift element.
These and other features and advantages are described in, or are apparent from, the following detailed description.